[analyzer] Fix incorrect doc references to CompilationUnitElement.

There is no `CompilationUnitElement` anymore. It has been replaced
with `LibraryFragment`.

This is a step towards enabling the `comment_references` lint in the
analyzer codebase.

Change-Id: I6a6a696403c7b1f060cd3ed5755f6575cd1ad918
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/450740
Auto-Submit: Paul Berry <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>

Author: stereotype441

pkg/analyzer/lib/src/dart/analysis/search.dart

@@ -1598,7 +1598,7 @@ class _IndexRequest {
     return results;
   }
 
-  /// Return the identifier of the [CompilationUnitElementIml] containing the
+  /// Return the identifier of the [LibraryFragmentImpl] containing the
   /// [element] in the [index] or `-1` if not found.
   int getUnitId(Element element) {
     var unitElement = getUnitElement(element);

pkg/analyzer/lib/src/dart/element/element.dart

@@ -3573,14 +3573,10 @@ abstract class FragmentImpl with _FragmentImplMixin implements Fragment {
   /// The unique integer identifier of this fragment.
   final int id = _NEXT_ID++;
 
-  /// The element that either physically or logically encloses this element.
+  /// The fragment that either physically or logically encloses this fragment.
   ///
-  /// For [LibraryElement] returns `null`, because libraries are the top-level
-  /// elements in the model.
-  ///
-  /// For [CompilationUnitElement] returns the [CompilationUnitElement] that
-  /// uses `part` directive to include this element, or `null` if this element
-  /// is the defining unit of the library.
+  /// For [LibraryFragment] returns `null`, because library fragments are the
+  /// top-level fragments in the model.
   @override
   FragmentImpl? enclosingFragment;